A servo control device that drives and controls an industrial machine, such as an industrial robot, a pressing device, and a line automation device, generates a command regarding a motion (a position and a speed) of a motor that drives a mechanical system, and controls the motion of the motor to follow the command. When the mechanical system has a low rigidity (such as when the mechanical system to be driven is an arm type or when a load machine is driven through a low-rigidity shaft or a decelerator), a deformation of a low-rigidity portion causes a difference between a motion of a distal end of the machine and the motion of the motor. Furthermore, when the mechanical system has a low rigidity, oscillations occur after the command changes to stop, or the like. Due to these causes, there is an error in the motion of the distal end of the machine relative to the command, and therefore control precision is degraded.
To solve the problems caused by the low rigidity as described above, there is a technology described in Patent Literature 1 to derive a second-order differential value of a position command value generated by a position command block with respect to time, and to multiply the second-order differential value by a gain constant in order to derive a correction value. A motor is controlled to follow a corrected position command value obtained by adding the correction value to an original position command value.
Furthermore, in the technology described in Patent Literature 1, it is described that a pseudo differential can be used instead of pure differential. Further, it is described that when a command speed shows a trapezoidal pattern, a time point at which command acceleration changes is extracted, and a preset time-series correction pattern is used to generate a correction value.
There is a case where the servo control device operates an industrial machine in a specific pattern in cycles that may not be necessarily regular. In such a case, a method including a command function unit, which is referred to as “electronic cam”, is used for example. This method uses a phase signal that indicates a phase in a cycle and that increases or decreases with the elapsed time. The command function unit uses an equation or refers to a data table based on the phase signal to generate a cyclic position command. Therefore, the position command having the same shape pattern is repeatedly generated to the phase, and the motor position follows the position command. There is a technology described in Patent Literature 2 to further correct a position command by a servo control device using the command function unit as described above, thereby enhancing a control precision.
The technology described in Patent Literature 2 is to correct a command for the purpose of improving the control precision of the servo control device that cyclically operates in the same pattern. In this technique, a command function unit (a position pattern generator) having a phase signal (a phase command value) as an input is used to generate a cyclic position command (a position pattern). To correct a delay of a follow-up control unit (a position control system), the control function unit uses the position command in which the phase is advanced, and also calculates a correction value by multiplying a second-order differential value or a third-order differential value of the position command with respect to time by a coefficient.
Furthermore, in the technology described in Patent Literature 2, it is described that whether the correction value described above is added is selected according to the magnitude of an absolute value of the second-order differential value or the third-order differential value of the position command, thereby suppressing a superimposition of noise on the position command.